In man, the allogeneic and autologous mixed leukocyte reactions (MLR) are elicited primarily by HLA-DR antigens. These reactions are characterized by proliferation of T lymphocytes as well as polyclonal activation of B cells and subsequent synthesis of immunoglobulin. We have shown that these reactions are regulated by suppressor T cells and soluble T suppressor factors (Tsf). Four types of MLR suppressor T cells have been studied: 1) HLA-D restricted suppressor cells of rare healthy individuals in whom suppression is mediated by antigen specific Tsf; 2) antigen non-specific suppressor cells induced, in vivo, by total lymphoid irradiation; 3) HLA-DR antigen specific, feedback suppressor cells induced in vitro by priming normal T cells in MLR; 4) T cells present in all normal subjects tested which regulate MLR induce immunoglobulin synthesis. To analyze the roles of lymphocyte subsets in these systems, monoclonal natibodies to T cellmarkers and HLA-DR antigen were produced. Studies with these antibodies indicate that each suppressor system is mediated by distinct subsets of T cells. Renewed funding will support efforts to identify surface antigens unique to suppressor cells, immunochemically characterize Tsf, identify the target of Tsf, localize genes controlling the induction and effector phases of suppression, and assess alterations of suppressor T cell functions in patients with malignant and autoimmune disorders. Subsets of suppressor T cells will be purified with existing monoclonal antibodies and cloned in the presence of Interleukin 2 for (a) analysis of cell surface molecules with one and two-dimensional gel electrophoresis and (b) production of additional monoclonal antibodies to suppressor cell markets. Clones suppressor cells will be fused with human T cell lines for production of hybrids secreting soluble suppressor factors which can be analyzed functionally and structurally. The proposed studies should permit more effective use of lymphocyte markers in analyzing genetic control of human disease. In addition, studies of experimental animals indicate that suppressor cells similar to those studied here influence the results of transplantation as well as susceptibility to malignant and autoimmune disorders. Therefore, this project should contribute ultimately to our ability to manipulate the human immune system to clinical advantage.